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authorAndy Grover <agrover@redhat.com>2018-07-27 15:51:57 -0700
committerMike Snitzer <snitzer@redhat.com>2018-07-30 11:49:08 -0400
commit63c8ecb6261abcb79191a264778e8dae222e67cf (patch)
treec40e72bd99cdac2f87fea33df6cf666740c0d0cf /Documentation/device-mapper
parent9ff07e7d634cb005e7c5dc5e2c28a06508eb4fbf (diff)
dm thin: include metadata_low_watermark threshold in pool status
The metadata low watermark threshold is set by the kernel. But the kernel depends on userspace to extend the thinpool metadata device when the threshold is crossed. Since the metadata low watermark threshold is not visible to userspace, upon receiving an event, userspace cannot tell that the kernel wants the metadata device extended, instead of some other eventing condition. Making it visible (but not settable) enables userspace to affirmatively know the kernel is asking for a metadata device extension, by comparing metadata_low_watermark against nr_free_blocks_metadata, also reported in status. Current solutions like dmeventd have their own thresholds for extending the data and metadata devices, and both devices are checked against their thresholds on each event. This lessens the value of the kernel-set threshold, since userspace will either extend the metadata device sooner, when receiving another event; or will receive the metadata lowater event and do nothing, if dmeventd's threshold is less than the kernel's. (This second case is dangerous. The metadata lowater event will not be re-sent, so no further event will be generated before the metadata device is out if space, unless some other event causes userspace to recheck its thresholds.) Signed-off-by: Andy Grover <agrover@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Diffstat (limited to 'Documentation/device-mapper')
-rw-r--r--Documentation/device-mapper/thin-provisioning.txt7
1 files changed, 6 insertions, 1 deletions
diff --git a/Documentation/device-mapper/thin-provisioning.txt b/Documentation/device-mapper/thin-provisioning.txt
index b8a57b9cec19..883e7ca5f745 100644
--- a/Documentation/device-mapper/thin-provisioning.txt
+++ b/Documentation/device-mapper/thin-provisioning.txt
@@ -281,7 +281,7 @@ ii) Status
<transaction id> <used metadata blocks>/<total metadata blocks>
<used data blocks>/<total data blocks> <held metadata root>
ro|rw|out_of_data_space [no_]discard_passdown [error|queue]_if_no_space
- needs_check|-
+ needs_check|- metadata_low_watermark
transaction id:
A 64-bit number used by userspace to help synchronise with metadata
@@ -328,6 +328,11 @@ ii) Status
thin-pool can be made fully operational again. '-' indicates
needs_check is not set.
+ metadata_low_watermark:
+ Value of metadata low watermark in blocks. The kernel sets this
+ value internally but userspace needs to know this value to
+ determine if an event was caused by crossing this threshold.
+
iii) Messages
create_thin <dev id>